Several generations of nickel and cobalt-base superalloys have been developed for turbine engines. However, despite superior mechanical and oxidation resistance properties, engine components manufactured of precipitation hardening superalloys are still prone to thermal fatigue cracking, oxidation, sulfidation and erosion.
For a repair of heavily damaged engine components Liburdi Engineering Ltd. developed and patented Liburdi Powder Metallurgy process (LPM™) first described in the U.S. Pat. No. 5,156,321 in 1992.
The LPM™ process is based on the application of a putty made of Mar M-247, Inconel 738 or other superalloys powder with organic binder to the repair area followed by sintering of the powder at temperatures exceeding 1000° C. to produce a porous compound that is metallurgicaly bonded to the substrate followed by liquid phase sintering of LPM™ to the repair area using low melting nickel or cobalt based alloys that forms in the repair area a deposit with superior mechanical and oxidation properties.
General Electric developed and introduced similar processes known as Activated Diffusion Healing (ADH) that was described in the article “Improving Repair Quality of Turbine Nozzles Using SA650 Braze Alloy”, by Wayne A. Demo, Stephen Ferrigno, David Budinger, and Eric Huron, Superalloys 2000, Edited by T. M. Pollock, R. D. Kissinger, R. R. Bowman, K. A. Green, M. McLean, S. Olson, and J. J. Schim, TM.5, The Minerals, Metals & Materials Society, 2000, pp. 713-720.
In ADH repair, a slurry is applied to the repair area. The slurry is made of a high melting point superalloy powder, usually the same composition as the alloy being repaired, and the ADH alloy, which has lower melting point that is achieved by adding boron (B) or silicon (Si) powders. The slurry is mixed together and suspended in standard organic-based brazing binders.
ADH alloys have achieved their low melting point primarily using boron. The boron level is balanced between a minimum that is required for braze flow, acceptable crack filling, and reasonably low braze process temperature on one side, against excessive deleterious impact on mechanical properties on the other side.
In both ADH and LPM processes the repair area includes a significant amount of low melting material, which make it extremely difficult to do following repairs or rework of any defects by fusion welding using conventional filler materials. As a result, for repair of even minor discontinuities LPM™ and ADH cycles have to be repeated increasing the cost of a repair and affecting properties of a parent material due to excessive diffusion of boron.
Joe Liburdi et al reported some progress in using of a GTAW welding with Inconel 625 filler wires for repairing of LPM™ materials in “Novel Approaches to the Repair of Vane Segments” at the International Gas Turbine and Aeroengine Congress and Exposition, Cincinnati, Ohio—May 24-27, 1993, Published by the American Society of Mechanical Engineers, 93-GT-230. However, the practical use of this method was limited due to inconsistency mostly because of a high melting temperature of Inconel 625 that exceeded a melting temperature of brazing materials used in the LPM process.
Additionally the direct GTAW welding on Inconel 738, Inconel 713, Rene 77 and other superalloys with a total content of aluminum and titanium exceeding 8% results in cracking of the heat affected zone (HAZ).
Previous attempts to produce crack free welds on Inconel 738 using standard filler wires were not successful in accordance with Banerjee K., Richards N I., and Chaturvedi M. C. “Effect of Filler Alloys on Heat Affected Zone Cracking in Pre-weld Heat Treated IN-738 LC Gas-Tungsten-Arc Welds”, Metallurgical and Materials Transactions, Volume 36A, July 2005, pp. 1881-1890. The effect of nickel based Hastelloy C-263 welding wire manufactured to Aerospace Materials Specification (AMS) 5966 and comprised of 0.4% Si among other alloying elements, and silicon and boron free nickel based AMS 5832 (also known as Inconel 718), AMS 5800 (Rene 41), AMS 5675 (FM-92) welding wires having different melting temperatures and chemical compositions on HAZ cracking was studied. It was shown that all samples produced using above mentioned filler materials exhibited extensive cracking leading to conclusion that the weld metal solidification temperature range did not correlate with susceptibility of Inconel 738 alloy to HAZ cracking.
To verify results above, the inventors within the scope of the current development conducted the evaluation of weldability of Inconel 738 with another group of welding materials that included standard AMS 5786 (Hastelloy W) and AMS 5798 (Hastelloy X) nickel based welding wires comprised numerous alloying elements including Si with the bulk contend of 1 wt. %, Haynes HR-160 nickel based welding wire with bulk content of silicon of 2.75 wt. % and other wilding wires wherein the bulk content of silicon varied from 0.05 wt. % to 2 wt. % similar to the alloy described in U.S. Pat. No. 2,515,185.
Regardless of the chemical composition, all welds produced using standard welding wires exhibited extensive intergranular micro cracking in the HAZ along the fusion line between base material and weld bead. The HAZ cracking in Inconel 738 was related to an incipient melting of low temperature eutectics, carbides and other precipitates along grain boundaries during welding followed by a propagation of cracks due to continuously raising level of tensile residual stresses in the HAZ during solidification and cooling of the weld bead.
Lack of low temperature eutectics and rapid cooling did not allow full crack back filling as it was shown by Alexandrov B. T., Hope A. T., Sowards J. W., Lippold J. C., and McCracken S. S, Weldability Studies of High-Cr, Ni-base Filler Metals for Power Generation Applications, Welding in the World, Vol. 55, n° 3/4, pp. 65 76, 2011 (Doc. IIW-2111, ex. Doc. IX-231.3-09). High melting temperatures of standard cobalt based welding materials with bulk content of Si up to 2.75% that exceeded the incipient melting temperature of Inconel 738 increased overheating and aggravated cracking in the HAZ. The post weld heat treatment (PWHT) of these welds resulted in an additional strain-aging cracking in the HAZ. Some cracks propagated into welds.
Therefore, currently only preheating of Inconel 738, Inconel 713, GTD 111, GTD 222, Rene 80 and other precipitation hardening polycrystalline and directionally solidified high gamma-prime superalloys, as well as Mar M247, Rene 80, CMSX 4, CMSX 10, Rene N5 and other single crystal materials to temperature exceeding 900° C. allows crack free welding. Methods using elevated temperatures for welding are taught in U.S. Pat. Nos. 5,897,801, 6,659,332 and CA 1207137. However, preheating of parts prior to welding increases cost and reduces productivity of welding operations.
Based on the foregoing a different approach to welding of superalloys is desirable which is able to minimize or eliminate the requirement for preheating and is able to minimize or eliminate HAZ cracking. We have found that by selectively segregating certain elements superior weldability of superalloys and properties of welded joints can be achieved by taking advantage of the differences in the melting (liquidus) and solidification (solidus) temperatures sometimes referred to as the melting or solidification range.
There are several types of composite welding wires know from prior art. For example, the composite weld wire disclosed in U.S. Pat. No. 5,569,546 is made by sintering powders namely a mixture by weight of about 50-90% Co base alloy and about 10-50% Ni base alloy consisting essentially of 1.5-2.5% B, 2-5% Al, 2-4% Ta, 1447% Cr, 8-12% Co, with the balance of Ni and incidental impurities in powder form. Boron is used as a melting point depressant allowing welding of articles manufactured of cobalt based alloys. However, boron reduces ductility of cobalt, nickel and iron based alloys. Therefore this patent teaches the manufacturing of this filler wire by sintering powders. This is a costly and time consuming process to carry out in practice.
The flux-cored welding wires and wires that are described in the AMS Handbook, Welding, Brazing and Soldering, Volume 6, pp. 719, FR2746046, CA 2442335, and CN 1408501 also belong to the general class of composite filler materials. The flux-cored welding wires and wires comprise a metal shell that is filled with different slag forming materials, arc stabilizers, dioxidizers, and metal powders. Composite core wire can be manufactured of unlimited variations of powders using high productivity processes. Unfortunately, diameters of these filler materials vary from 4 to 8 mm that does not allow using them for repair and manufacturing of turbine engine components with wall thickness from 1 to 3 mm.
The bimetal composite welding wire has a good metallurgical bonding between the core and shell but it can be manufactured by drawing as per RU 2122908 using only high ductility materials such as copper and stainless steel.
The composite copper plated welding wire is disclosed in JP 2007331006, JP 2006281315, JP 62199287 and KR 20090040856. These wires have different chemical composition and are available on the global market from different suppliers. However, copper drastically reduces the service temperature of welded joints of nickel based superalloys. Therefore, they are not suitable for repair of turbine engine components.
The silver-copper coating of welding wires as per CN 1822246 due to metallurgical peculiarities of interaction with nickel and cobalt based superalloys, also are not suitable for weld of turbine engine components as well.
Titanium surface coating as per CN 101407004, CN 201357293 and JP 2007245185 is not effective for reducing the melting temperature of filler materials.
Coating of welding wire with active agent made of MnCl2, CaCl2, MnO2, and ZnO as per CN 101244489 is not effective for HAL crack prevention of welding of precipitation hardening superalloys.
Composite welding wires and wires as per CN 1822246, RU 2415742 and RU 2294272 with inner and outer coatings containing activating fluxes aimed to reduce moisture absorption. These composite wires may also include metal coating. However, these filler wires are not able to produce defect free welds on precipitation hardening superalloys due to the high melting temperature and overheating of the heat affected zone due to hygroscopic components that do not reduce the melting temperature.
Therefore, due to technological difficulties in manufacturing and use of known filler wires, there is little to no availability of filler wires or wires which include a high content of melting point depressant for weld repair of turbine engine components by GTAW welding. Additionally currently no filler wires are available to produce crack free welds on Inconel 738 and other high gamma prime superalloys without preheating. Only AMS 4777 is commercially available in form of brazing cast rods. However, due to the low melting temperature of these rods, they are not suitable for repair of high pressure turbine (HPT) engine components.
Based on the foregoing it would advantageous to develop an effective composite welding wire for fusion welding and TIG (GTAW) braze-welding on precipitation hardening superalloys that are prone to cracking in the HAL and that were exposed previously to brazing, LPM™ or ADH repairs.